]> Cisco Systems, Inc.

cschmutz@cisco.com

Cisco Systems, Inc.

zali@cisco.com

Airtel India

Praveen.Maheshwari@airtel.com

Ciena

rrokui@ciena.com

Nokia

andrew.stone@nokia.com

This document describes how Segment Routing (SR) policies can be used to satisfy the requirements for bandwidth, end-to-end recovery and persistent paths within a SR network. The association of two co-routed unidirectional SR Policies satisfying these requirements is called "circuit-style" SR Policy (CS-SR Policy).

Introduction IP services typically leverage ECMP and local protection. However packet transport services (commonly referred to as "private lines") that are delivered via pseudowires such as , , , and for example, require:

• Persistent end-to-end bidirectional traffic engineered paths that provide predictable and identical latency in both directions
• A requested amount of bandwidth per path that is assured irrespective of changing network utilization other services
• Fast end-to-end protection and restoration mechanisms
• Monitoring and maintenance of path integrity
• Data plane remaining up while control plane is down

Such a "transport centric" behavior is referred to as "circuit-style" in this document. This document describes how Segment Routing (SR) Policies and adjacency segment identifiers (adjacency-SIDs) defined in the SR architecture together with a centralised controller such as a stateful Path Computation Element (PCE) can be used to satisfy those requirements. It includes how end-to-end recovery and path integrity monitoring can be implemented. A "Circuit-Style" SR Policy (CS-SR Policy) is an association of two co-routed unidirectional SR Policies satisfying the above requirements and allowing for a single SR network to carry both typical IP (connection-less) services and connection-oriented transport services.

Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology

• BSID : Binding Segment Identifier
• CS-SR : Circuit-Style Segment Routing
• DWDM : Dense Wavelength Division Multiplexing
• ID : Identifier
• LSP : Label Switched Path
• LSPA : LSP Attributes
• NRP : Network Resource Partition
• OAM : Operations, Administration and Maintenance
• OF : Objective Function
• PCE : Path Computation Element
• PCEP : Path Computation Element Communication Protocol
• PT : Protection Type
• SID : Segment Identifier
• SLA : Service Level Agreement
• SDH : Synchronous Digital Hierarchy
• SONET : Synchronous Optical Network
• SR : Segment Routing
• STAMP : Simple Two-Way Active Measurement Protocol
• TI-LFA : Topology Independent Loop Free Alternate
• TLV : Type Length Value

Reference Model The reference model for CS-SR Policies follows the SR architecture and SR Policy architecture and is depicted in .

Circuit-style SR Policy Reference Model | controller |<------------+ | +----------------+ | PCEP/BGP/config PCEP/BGP/config | | v <<<<<<<<<<<<<< CS-SR Policy >>>>>>>>>>>>> v +-------+ +-------+ | |=========================================>| | | A | SR Policy from A to Z | Z | | |<=========================================| | +-------+ SR Policy from Z to A +-------+ ]]>

Given the nature of CS-SR Policies, paths are computed and maintained by a centralized entity providing a consistent simple mechanism for initializing the co-routed bidirectional end-to-end paths, performing bandwidth allocation control, as well as monitoring facilities to ensure SLA compliance for the live of the CS-SR Policy. CS-SR Policies can be instantiated in the headend routers by using PCEP or BGP as a communication protocol between the headend routers and the central controller or by configuration.

• When using PCEP as the communication protocol, the controller is a stateful PCE as defined in . When using SR-MPLS , PCEP extensions defined in are used. When using SRv6 , PCEP extensions defined in are used.
• When using BGP as the communication protocol, the BGP extensions defined in are used.
• When using configuration, a appropriate YANG model such as can be used.

To satisfy the requirements of CS-SR Policies, each link in the topology MUST have:

• An adjacency-SID which is:
  • Statically configured or auto-generated, but persistent: to ensure that its value does not change after an event that may cause dynamic states to change (e.g. router reboot).
  • Non-protected: to avoid any local TI-LFA protection to happen upon interface/link failures.
• The bandwidth available for CS-SR Policies specified.
• A per-hop behavior ( or ) that ensures that the specified bandwidth is always available to CS-SR Policies independent of any other traffic.

When using link bundles (i.e. ), parallel physical links are only represented via a single adjacency. To ensure deterministic traffic placement onto physical links, an adjacency-SID SHOULD be assigned to each physical link (aka member-link) (, ). Similarly, the use of adjacency-SIDs representing parallel adjacencies SHOULD also be avoided. When using SR-MPLS , existing IGP extensions defined in and and BGP-LS defined in can be used to distribute the topology information including those persistent and unprotected adjacency-SIDs. When using SRv6 , the IGP extensions defined in and and BGP-LS extensions in apply. CS-SR Policy state reporting by the headend routers back to the central controller is essential to confirm success or failure of the instantiation and making the controller aware of any state changes throughout the lifetime of the CS-SR Policy in the network.

• When using PCEP for CS-SR policy instantiation, reporting is done using PCEP procedures of .
• When using BGP for CS-SR policy instantiation, reporting using BGP-LS procedures has the benefit of both instantiation and state reporting being done over a single protocol (BGP), but reporting can also be done using an appropriate YANG model such as .
• When using configuration for CS-SR policy instantiation, state reporting could be done using an appropriate YANG model such as or BGP-LS procedures .

Managing Bandwidth In a network, resources are represented by links of certain bandwidth. In a circuit switched network such as SONET/SDH, OTN or DWDM resources (timeslots or a wavelength) are allocated for a provisioned connection at the time of reservation even if no communication is present. In a packet switched network, resources are only allocated when communication is present, i.e. packets are to be sent. This allows for the total reservations to exceed the link bandwidth as well in general for link congestion. To satisfy the bandwidth requirement for CS-SR Policies it must be ensured that packets carried by CS-SR Policies can always be sent up to the reserved bandwidth on each hop along the path. This is done by:

• Firstly, CS-SR Policy bandwidth reservations per link must be limited to equal or less than the physical link bandwidth.
• Secondly, ensuring traffic for each CS-SR Policy is limited to the bandwidth reserved for that CS-SR Policy by traffic policing or shaping and admission control on the ingress of the pseudowire.
• Thirdly, ensuring that during times of link congestion only non-CS-SR Policy traffic is being buffered or dropped.

For the third step several approaches can be considered:

• Allocate a dedicated physical link of bandwidth P to CS-SR Policies and allow CS-SR reservations up to bandwidth C. Consider bandwidth N allocated for network control, ensure that P - N >= C.
• Allocate a dedicate logical link (i.e. 801.q VLAN on ethernet) to CS-SR Policies on a physical link of bandwidth P. Limit the total utilization across all other logical links to bandwidth O by traffic policing or shaping and ensure that P - N - O >= C.
• Allocate a dedicated Diffserv codepoint to map traffic of CS-SR Policies into a specific queue not used by any other traffic.
• Use of dedicated persistent unprotected adjacency-SIDs that are solely used by CS-SR traffic, managed by network design and policy (which is outside the scope of this document). These dedicated SIDs used by CS-SR Policies MUST NOT be used by features such as TI-LFA for defining the repair path and microloop avoidance for defining the loop-free path.

The approach of allocating a Diffserv codepoint can leverage any of the following Per-Hop Behavior (PHB) strategies below, where P is the bandwidth of a physical link, N is the bandwidth allocated for network control and C is the bandwidth reserved for CS-SR policies:

• Use a Assured Forwarding (AF) class queue for CS-SR Policies and limit the total utilization across all other queues to bandwidth O by traffic policing or shaping and ensure that P - N - O >= C.
• Use a Expedited Forwarding (EF) class queue for CS-SR Policies and limit the total utilization across all other EF queues of higher or equal priority to bandwidth O by traffic policing or shaping and ensure that P - N - O >= C.
• Use a Expedited Forwarding (EF) class queue for CS-SR Policies with a priority higher than all other EF queues and limit the utilization of the CS-SR Policy EF queue by traffic policing to C <= P - N.

The use of a dedicated Diffserv codepoint for CS-SR traffic requires the marking of all traffic steered into CS-SR Policies on the ingress with that specific codepoint consistently across the domain. In addition, the headends MAY measure the actual bandwidth utilization of a CS-SR Policy to raise alarms when bandwidth utilization thresholds are passed or to request the reserved bandwidth to be adjusted. Using telemetry collection the alarms or bandwidth adjustments can also be triggered by the controller.

CS-SR Policy Characteristics A CS-SR Policy has the following characteristics:

• Requested bandwidth: bandwidth to be reserved for the CS-SR Policy
• Bidirectional co-routed: a CS-SR Policy between headends A and Z is an association of an SR Policy from A to Z and an SR Policy from Z to A following the same path(s)
• Deterministic and persistent paths: segment lists with strict hops using unprotected adjacency-SIDs.
• Not automatically recomputed or reoptimized: the segment list of a candidate path MUST NOT change automatically to a segment list representing a different path (for example upon topology change).
• More than one candidate paths in case of protection/restoration:
  • Following the SR Policy architecture, the highest preference valid path is carrying traffic.
  • Depending on the protection/restoration scheme (), lower priority candidate paths
    • may be pre-computed.
    • may be pre-programmed.
    • may have to be disjoint.
• It is RECOMMENDED that candidate paths only contain one segment list to avoid asymmetrical routing due to independent load balancing across segment lists on each headend
• Continuity check and performance measurement are activated on each candidate path () and performed per segment-list.

CS-SR Policy Creation

Policy Creation when using PCEP

PCC-initiated Mode Considering the scenario illustrated in a CS-SR Policy between headends A and Z is instantiated by configured a SR Policy on both headend A (with Z as endpoint) and headend Z (with A as endpoint). Both headend routers A and Z act as PCC and delegate path computation to the PCE using PCEP with the procedure described in . For SR-MPLS the extensions defined in are used. And SRv6 specific extensions are defined in . The PCRpt message sent from the headends to the PCE SHOULD contain the following parameters:

• BANDWIDTH object (Section 7.7 of ) : to indicate the requested bandwidth
• LSPA object (section 7.11 of ) : to indicate that no local protection requirements
  • L flag set to 0 : no local protection
  • E flag set to 1 : protection enforcement (section 5 of )
• ASSOCIATION object () :
  • Type : Double-sided Bidirectional with Reverse LSP Association ()
  • Bidirectional Association Group TLV () :
    • R flag is always set to 0 (forward path)
    • C flag is always set to 1 (co-routed)

If the SR Policies are configured with more than one candidate path, a PCRpt message MUST be sent per candidate path. Each PCRpt message does include the "SR Policy Association" object (type 6) as defined in to make the PCE aware of the candidate path belonging to the same policy. The signaling extensions described in are used to ensure that:

• Path determinism is achieved by the PCE only using segment lists representing a strict hop by hop path using unprotected adjacency-SIDs.
• Path persistency across events that may cause dynamic states to change in the network (e.g. router reboot) is achieved by the PCE only including statically configured adjacency-SIDs in its path computation response.
• Persistency across network changes is achieved by the PCE not performing periodic or network event triggered re-optimization.

Bandwidth adjustment can be requested after initial creation by signaling both requested and operational bandwidth in the BANDWIDTH object but the PCE MUST NOT respond with a changed path. As discussed in section 3.2 of it may be necessary to use load-balancing across multiple paths to satisfy the bandwidth requirement of a candidate path. In such a case the PCE will notify the headends A and Z to install multiple segment lists using the signaling procedures described in section 5.3 of . The candidate paths of the CS-SR Policy are reported and updated following PCEP procedures of .

PCE-initiated Mode The CS-SR Policy can be instantiated in the network between headends A and Z by a PCE using PCE-initiated procedures defined in . For PCE-initiated procedures no SR Policy configuration is required on the headends A and Z acting as PCC. The PCE requests the headends A and Z to initiate the candiate paths of the CS-SR Policy by sending a PCInitiate message. The PCInitiate message contains the same Bandwidth, LSPA, and ASSOCIATION objects used in PCC-initiated mode. Following initiation, the candidate paths of the CS-SR Policy are reported and updated following PCEP procedures of and share the same behavior as the PCC-initiated mode. Connectivity verification and performance measurement is enabled via local policy configuration on the headends, as there is no standard signaling mechanism available.

Policy Creation when using BGP Again, considering the scenario illustrated in , instead of configuring SR Policies on both headend A (with Z as endpoint) and headend Z (with A as endpoint), a CS-SR Policy between A and Z is instantiated by a request (e.g. application API call) to the controller. The controller performs path computation and advertises the corresponding SR Policies to the headend routers via BGP. To instantiate the SR Policies in headends A and Z the BGP extensions defined in are used. No signaling extensions are required for the following:

• Path determinism is achieved by the controller only computing strict paths and only including unprotected adjacency-SIDs in segment lists. Loose hops SHOULD NOT be used.
• Path persistency across events that may cause dynamic states to change in the network (e.g. router reboot) is achieved by the controller only including manually configured adjacency-SIDs in its path computation response.
• Persistency across network changes is achieved by the controller not performing periodic or network event triggered re-optimization.

If there are more than one candidate paths per SR Policy required, multiple NLRIs with different distinguisher values (see section 2.1 of ) have to be included in the BGP UPDATE message. To achieve load-balancing across multiple paths to satisfy the bandwidth requirement of a candidate path, multiple Segment List Sub-TLVs have to be included in the SR Policy Sub-TLV. See section 2.1 of . The candidate paths of a CS-SR Policy are updated by the controller sending another BGP UPDATE message to the headends A and Z. The headends A and Z can report the CS-SR Policy candidate path state back to the controller via BGP-LS using the extension defined in . Alternatively, CS-SR Policy candidate path state can be gathered using an appropriate YANG model such as . Connectivity verification and performance measurement is enabled via local policy configuration on the headends, as there is no standard signaling mechanism available..

Maximum SID Depth Constraint The segment lists used by CS-SR Policy candidate paths are constrained by the maximum number of segments a router can impose onto a packet. When using SR-MPLS this constraint is called "Base MPLS Imposition MSD" and is advertised via IS-IS , OSPF , BGP-LS and PCEP . When using SRv6 this constraint is called "SRH Max H.encaps MSD" and is advertised via IS-IS , OSPF , BGP-LS and PCEP . The MSD constraint is typically resolved by leveraging a segment list reduction technique, such as using Node SIDs and/or Binding SIDs (BSIDs) (SR architecture ) in a segment list, which represents one or many hops in a given path. As described in , adjacency-SIDs without local protection are used in CS-SR Policies to ensure that there is no per-hop ECMP, no localized rerouting due to topological changes, and no invocation of localized protection mechanisms, as the alternate path may not be providing the desired SLA. If a CS-SR Policy path requires segment list reduction, a SR Policy can be programmed in a transit node, and its BSID can be used in the segment list of the CS-SR Policy, if the following requirements are met:

• The transit SR Policy is unprotected, hence only has one candidate path.
• The transit SR Policy follows the rerouting and optimization characteristics defined in which implies the segment list of the candidate path MUST only use unprotected adjacency-SIDs.

This ensures that traffic for CS-SR Policies using a BSID does not get locally rerouted due to topological changes or locally protected due to failures. A transit SR Policy may be pre-programmed in the network or automatically injected in the network by a PCE.

CS-SR Policy Deletion

Policy Deletion when using PCEP When using PCC-initiated mode, the headends A and Z send a PCRpt message with the R flag set to 1 to inform the PCE about the deletion of a candidate path. When using PCE-initiated mode, the PCE does send a PCInitiate message to the headends A and Z and to instruct them to delete a candidate path.

Policy Deletion when using BGP The controller is using the withdraw procedures of to instruct headends A and Z to delete a candidate path.

Recovery Schemes Various recovery (protection and restoration) schemes can be implemented for a CS-SR Policy. As described in , there is a subtle distinction between the terms "protection" and "restoration" based on the resource allocation done during the recovery path establishment. The same definitions apply for CS-SR Policy recovery schemes, wherein:

• Protection: another candidate path is computed and fully established in the data plane and ready to carry traffic.
• Restoration: a candidate path may be computed and may be partially established but is not ready to carry traffic.

The term "failure" is used to represent both "hard failures" such complete loss of connectivity detected by continuity check described in or degradation, i.e., when the packet loss ratio increased beyond a configured acceptable threshold.

Unprotected In the most basic scenario, no protection or restoration is required. The CS-SR Policy has only one candidate path. In case of a failure along the path the CS-SR Policy will go down and traffic will not be recovered. Typically, two CS-SR Policies are deployed either within the same network with disjoint paths or in two separate networks and the overlay service is responsible for traffic recovery. As soon as the failure(s) that brought the candidate path down are cleared, the candidate path is activated, traffic is sent across it and state is reported accordingly.

Setup When using PCEP, the single candidate path is established using the procedures defined in , activated and is carrying traffic. A PCRpt message is sent from the headends A and Z to the PCE with the O field in the LSP object set to 2 to indicate the candidate path is active and carrying traffic. When using BGP, the single candidate path is established using the procedures defined in , activated and is carrying traffic. A BGP-LS update is sent from the headends A and Z to the controller with the SR Candidate Path State TLV of the SR Policy Candidate Path NLRI having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• A-Flag set to 1 to indicate the candidate path is active and is carrying traffic.

Failure When using PCEP, a PCRpt message is sent by the headends A and Z to the PCE with O field in LSP object changed from 2 to 0, to indicate the candidate path is no longer active and not carrying traffic. When using BGP, a BGP-LS update is sent by the headends A and Z to the controller with the SR Policy Candidate Path NLRI of the candidate path and the SR Candidate Path State TLV having the A-Flag cleared to indicate the candidate path is no longer active and not carrying traffic.

Reversion When using PCEP, a PCRpt message is sent by the headends A and Z to the PCE with O field in LSP object is set to 2, to indicate this candidate path is active again and traffic is sent across it. When using BGP, a BGP-LS update is sent by the headends A and Z to the controller with the SR Policy Candidate Path NLRI of the candidate path and the SR Candidate Path State TLV having the A-Flag change to 1 to indicate the candidate path is active again and traffic is sent across it.

1:1 Protection For fast recovery against failures the CS-SR Policy has two candidate paths. Both paths are established but only the candidate with higher preference is activated and is carrying traffic. The second candidate path is programmed as backup in the forwarding plane as described in . Upon a failure impacting the candidate path with higher preference carrying traffic, the candidate path with lower preference is activated immediately and traffic is now sent across it. Protection switching is bidirectional. As described in , both headends will generate and receive their own loopback mode test packets, hence even a unidirectional failure will always be detected by both headends without protection switch coordination required. Two cases are to be considered when the failure(s) impacting a candidate path with higher preference are cleared:

• Revertive switching: re-activate the higher preference candidate path and start sending traffic over it.
• Non-revertive switching: do not activate the higher preference candidate path and keep sending traffic via the lower preference candidate path.

Setup When using PCEP, the two candidate paths are established using the procedures defined in . The candidate path with higher preference is activated and is carrying traffic. When using PCC-initiated mode, appropriate diverse routing of the candidate path with lower preference from the candidate path with higher preference can be requested by the headends A and Z from the PCE by using the "Disjointness Association" object (type 2) defined in in the PCRpt messages. The disjoint requirements are communicated in the "DISJOINTNESS-CONFIGURATION TLV"

• L bit set to 1 for link diversity
• N bit set to 1 for node diversity
• S bit set to 1 for SRLG diversity
• T bit set to enforce strict diversity

The P bit may be set for the candidate path with higher preference to allow for finding the best path for it that does satisfy all constraints without considering diversity to the candidate path with the lower preference. The "Objective Function (OF) TLV" as defined in section 5.3 of may also be added to minimize the common shared resources. When using PCE-initated mode, no signaling of diversity requirements between headends and the PCE is required. A PCRpt message for the candidate path with higher preference is sent by the headends A and Z to the PCE with the O field in the LSP object set to 2 to indicate this candidate path is active and carrying traffic. Further, a PCRpt message for the candidate path with the lower preference is sent with the O field in the LSP object set to 1 to indicate the candidate path is signaled but not carrying traffic. When using BGP, the two candidate paths are established using the procedures defined in . The candidate path with higher preference is activated and is carrying traffic. When using BGP, the controller is already aware of the disjoint requirements and does consider them while computing both paths. Two NLRIs with different distinguisher values and different preference values are included in the BGP UPDATE sent by the controller to the headend routers. A BGP-LS update is sent by the headends A and Z to the controller with a SR Policy Candidate Path NLRI for the candidate path with higher preference where the SR Candidate Path State TLV is having the

• C-Flag set to 1 to indicate that candidate path was provisioned by the controller, and
• A-Flag set to 1 to indicate the candidate path is active and is carrying traffic.

Further, another SR Policy Candidate Path NLRI for the candidate path with lower preference where the SR Candidate Path State TLV is included having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• B-Flag set to 1 to indicate the role of backup path.

Failure When using PCEP, a PCRpt message for the higher preference candidate path is sent by the headends A and Z to the PCE with the O field changed from 2 to 0 to indicate that the candidate path is no longer active and not carrying traffic anymore. Further, a PCRpt message for the lower preference candidate path is sent with the O field changed from 1 to 2 to indicate that the candidate path got activated and is carrying traffic. When using BGP, a BGP-LS update is sent by the headends A and Z to the controller with the SR Policy Candidate Path NLRI for the higher preference candidate path with the SR Candidate Path State TLV having the A-Flag cleared to indicate that the candidate path is no longer active and not carrying traffic anymore. Further, the SR Policy Candidate Path NLRI for the lower preference candidate path with the SR Candidate Path State TLV having the B-Flag cleared and A-Flag set to 1 is included in the BGP-LS update to indicate that the candidate path got activated and is carrying traffic.

Reversion When using PCEP, for revertive switching a PCRpt message for the recovered higher preference candidate path is sent by the headends A and Z to the PCE with the O field changed from 0 to 2 to indicate the higher preference candidate path got re-activated and is carrying traffic. Further, a PCRpt message is sent for the lower preference candidate path with the O field changed from 2 to 1 to indicate that the lower preference candidate path is no longer active but signaled. For non-revertive switching only a PCRpt message for the recovered higher preference candidate path with the O field set to 1 is sent to indicate that the higher preference candidate path got signaled but is not active. When using BGP, for revertive switching a BGP-LS update is sent by the headends A and Z to the controller with the SR Policy Candidate Path NLRI for the recovered higher preference candidate path with the SR Candidate Path State TLV having the A-Flag set to 1 to indicate the higher preference candidate path got re-activated and is carrying traffic. Further, the SR Policy Candidate Path NLRI for the lower preference candidate path with the SR Candidate Path State TLV having the A-Flag cleared and B-Flag set to 1 is included in the BGP-LS update to indicate that the lower preference candidate path is no longer active but signaled. For non-revertive switching only a BGP-LS update with a SR Policy Candidate Path NLRI for the higher preference candidate path with the SR Candidate Path State TLV having the B-Flag set to 1 is sent to indicate that the higher preference candidate path got signaled but is not active.

Restoration

1+R Restoration Similarly to 1:1 protection described in , in this recovery scheme the CS-SR Policy has two candidate paths. To avoid pre-allocating protection bandwidth by the controller ahead of failures, but still being able to recover traffic flow over an alternate path through the network in a deterministic way (maintaining the required bandwidth commitment), the second candidate path with lower preference is established "on demand" and activated upon failure of the first candidate path. As soon as failure(s) that brought the first candidate path down are cleared, the second candidate path is getting torn down and traffic is reverted back to the first candidate path. Restoration and reversion behavior is bidirectional. As described in , both headends use continuity check in loopback mode and therefore, even in case of unidirectional failures, both headends will detect the failure or clearance of the failure and switch traffic away from the failed or to the recovered candidate path.

Setup The first candidate path is set up as described in .

Failure When using PCEP, the second candidate path with lower preference is established using the procedures in , activated and traffic is sent across it. A PCRpt message for the lower preference candidate path is sent by the headends A and Z to the PCE with the O field set to 2 to indicate this candidate path is active and carrying traffic. Further, a PCRpt message for the higher preference candidate path is sent to the PCE with the O field changed from 2 to 0 to indicate this candidate path is no longer active. When using BGP, the second candidate path with lower preference is established using the procedures defined in . A BGP-LS update with the SR Policy Candidate Path NLRI for the lower preference candidate path is sent by the headends A and Z to the controller with the SR Candidate Path State TLV having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• A-Flag set to 1 to indicate the candidate path is active and is carrying traffic.

Further, the SR Policy Candidate Path NLRI for the higher preference candidate path is included with the SR Candidate Path State TLV having the A-Flag cleared, to indicate that the candidate this path is no longer active and not carrying traffic anymore.

Reversion When using PCEP, the second candidate path with lower preference is torn down using the procedures in . A PCRpt message for the remaining candidate path is sent by the headends A and Z to the PCE with O field in LSP object is set to 2, to indicate this candidate path is active and traffic is sent across it. When using BGP, the second candidate path with lower preference is torn down by using the procedures in . A BGP-LS update with the SR Policy Candidate Path NLRI for the remaining candidate path is sent to the controller with the SR Candidate Path State TLV having the

• A-Flag set to 1 to indicate the candidate path became active and is carrying traffic again.

1:1+R Restoration For further resiliency in case of multiple concurrent failures that could bring down both candidate paths of 1:1 protection described in , a third candidate path with a preference lower than the other two candidate paths (in this section referred to as first and second candidate path) is added to the CS-SR Policy to enable restoration. There are two possible operating models:

• R established upon double failure

• As in , to avoid pre-allocating additional bandwidth by the controller ahead of failures, the third candidate path may only be requested when both candidate paths are affected by failures.

• As soon as either the first or second candidate path recovers, traffic will be reverted and the third candidate path MUST be torn down.

• R pre-established after single failure

• Alternatively, the third candidate path can also be requested and pre-computed already whenever either the first or second candidate path go down with the downside of more bandwidth being set aside ahead of time. When doing so, the third candidate path MUST be computed diverse to the still operational candidate path.

• The third candidate path will get activated and carry traffic when further failures lead to both the first and second candidate path being down.

• As long as either the first or the second candidate path is active, the third candidate path is kept, updated (if needed) to ensure diversity to the active candidate path and is not carrying traffic.

• Once both, the first and the second candidate path have recovered, the third candidate path is torn down.

Again, restoration and reversion behavior is bidirectional. As described in , both headends use continuity check in loopback mode and therefore even in case of unidirectional failures both headends will detect the failure or clearance of the failure and switch traffic away from the failed or to the recovered candidate path.

R established upon double failure

Setup The first and second candidate path are set up as described in

Failure As failure(s) have brought down both the first and second candidate path, a third candidate path with lowest preference is established, activated and traffic is sent across it immediately to restore traffic. When using PCEP, the third candidate path is established using the procedures in . A PCRpt message for the third candidate path is sent by the headends A and Z to the PCE with the O field set to 2 to indicate this candidate path is active and carrying traffic. Further, a PCRpt message for both the first and second candidate path is sent to the PCE with the O field set to 0 to indicate the candidate paths are no longer active and are not carrying traffic. When using BGP, the third candidate path is established using the procedures defined in . A BGP-LS update is sent by the headends A and Z to the controller with a SR Policy Candidate Path NLRI for the third candidate path with the SR Candidate Path State TLV having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• A-Flag set to 1 to indicate the candidate path is active and is carrying traffic.

Further, the SR Policy Candidate Path NLRIs for the first and second candidate path are also included with the SR Candidate Path State TLV having the A-Flag and B-Flag cleared to indicate that those candidate paths are no longer active or backup and are not carrying traffic.

Reversion When using PCEP, the third candidate path is torn down using the procedures in . A PCRpt message for the recovered candidate path is sent by the headends A and Z to the PCE with O field in LSP object is set to 1, to indicate this candidate path is signaled but is not carrying traffic. When using BGP, the third candidate path is torn down by using the procedures in . A BGP-LS update with the SR Policy Candidate Path NLRI for the covered candidate path is sent by the headends A and Z to the controller with the SR Candidate Path State TLV having the B-Flag set to 1 to indicate the candidate path became backup and is not carrying traffic.

R pre-established after single failure

Setup The first and second candidate path are set up as described in

Failure As a failure brought either the first or the second candidate path down, a third candidate path is established, but is not activated and is not carrying traffic. When using PCEP, a PCRpt message for the third candidate path is sent by the headends A and Z to the PCE with the O field set to 1 to indicate this candidate path is signaled but not carrying traffic. Further, a PCRpt message for the failed candidate path is sent to the PCE with the O field set to 0 to indicate this candidate path is no longer active and not carrying traffic. When using BGP, a BGP-LS update is sent by the headends A and Z to the controller with a SR Policy Candidate Path NLRI for the third candidate path with the SR Candidate Path State TLV having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• B-Flag set to 1 to indicate the role of backup path.

Further, the SR Policy Candidate Path NLRIs for the failed candidate path is also included with the SR Candidate Path State TLV having the A-Flag and B-Flag cleared to indicate that the candidate path is no longer active or backup and is not carrying traffic. Whenever later a failure happens, that leads to both the first and second candidate path to be down, the third candidate path gets activated and traffic is sent across it. When using PCEP, a PCRpt message for the third candidate path is sent by the headends A and Z to the PCE with the O field set to 2 to indicate this candidate path is active and carrying traffic. Further, a PCRpt message for both the failed candidate path is sent to the PCE with the O field set to 0 to indicate the candidate path is no longer active and is not carrying traffic. When using BGP, a BGP-LS update is sent by the headends A and Z to the controller with a SR Policy Candidate Path NLRI for the third candidate path with the SR Candidate Path State TLV having the

• C-Flag set to 1 to indicate the candidate path was provisioned by the controller, and
• A-Flag set to 1 to indicate the candidate path is active and is carrying traffic.

Further, the SR Policy Candidate Path NLRI for the failed candidate path is also included with the SR Candidate Path State TLV having the A-Flag cleared to indicate that the candidate path is no longer active and is not carrying traffic.

Reversion When transitioning from a state where both the first and second candidate path being down to a state where either of them is recovered. The third candidate path MAY be updated to ensure it is diverse to the active candidate path. When using PCEP, the third candidate path is updated following PCEP procedures of . When using BGP, the controller is sending a new BGP update with the SR Policy Candidate Path NLRI containing the new path. When both the first and second candidate path have recovered, the third candidate MUST be torn down and the reversion procedures of MUST be followed. When using PCEP, the third candidate path is torn down using the procedures in . When using BGP, the third candidate path is torn down by using the procedures in .

Operations, Administration, and Maintenance (OAM)

Continuity Check The continuity check for each segment list on both headends MAY be done using

• Simple Two-Way Active Measurement Protocol (STAMP) in loopback measurement mode as described in section 6 and the session state described in section 11 of for SR-MPLS and for SRv6.
• Bidirectional Forwarding Detection (BFD) .
• Seamless BFD (S-BFD) .

The use of STAMP is RECOMMENDED as it leverages a single protocol for both continuity check and performance measurement (see of this document) and allows for a single session to be used, depending on the desired performance measurement session mode (two-way described in section 4, one-way described in section 5 or loopback described in section 6 of for SR-MPLS and for SRv6). As the STAMP test packets are including both the segment list of the forward and reverse path, standard segment routing data plane operations will make those packets get forwarded along the forward path to the tailend and along the reverse path back to the headend. To be able to send STAMP test packets for loopback measurement mode, the STAMP Session-Sender (i.e., the headend) needs to acquire the segment list information of the reverse path:

• When using PCEP, the headend forms the bidirectional SR Policy association using the procedure described in and receives the information about the reverse segment list from the PCE as described in section 4.5 of
• When using BGP, the controller does inform the headend routers about the reverse segment list using the Reverse Segment List Sub-TLV defined in section 4.1 of .

For cases where multiple segment lists are used by a candidate path, the headends will declare a candidate path down after continuity check has failed for one or more segment lists because the bandwidth requirement of the candidate path can no longer be met.

Performance Measurement Assuming a single STAMP session in loopback mode is used for continuity check and performance measurement, the round-trip delay can be measured and the round-trip loss can be estimated as described in section 8 of for SR-MPLS and for SRv6. Considering that candidate paths are co-routed, the delay in the forward and reverse direction can be assumed to be similar. Under this assumption, one-way delay can be derived by dividing the round-trip delay by two.

Candidate Path Validity Verification A stateful PCE/controller is in sync with the headend routers in the network topology and the CS-SR Policies provisioned on them. As described in a path MUST NOT be automatically recomputed by the controller after or optimized for topology changes unless it is a restoration path. However, there may be a requirement for the stateful PCE/controller to tear down a path if the path no longer satisfies the original requirements, such as insufficient bandwidth, diversity constraint no longer met or latency constraint exceeded and only the stateful PCE/controller can detect this and not the headend routers themselves. For a CS-SR Policy configured with multiple candidate paths, a headend may switch to another candidate path if the stateful PCE/controller decided to tear down the active candidate path.

External Commands External commands are typically issued by an operator to control the candidate path state of a CS-SR Policy using the management interface of:

• Headends: When the CS-SR Policy was instantiated via configuration or PCEP PCC-initiated mode
• PCE/controller: When the CS-SR Policy was instantiated via BGP or PCEP PCE-initiated mode

Candidate Path Switchover It is very common to allow operators to trigger a switch between candidate paths even if no failure is present, e.g., to proactively drain a resource for maintenance purposes. A operator triggered switching request between candidate paths on a headend is unidirectional and SHOULD be requested on both headends to ensure co-routing of traffic.

Candidate Path Re-computation While no automatic re-optimization or pre-computation of CS-SR Policy candidate paths is allowed as specified in , network operators trying to optimize network utilization may explicitly request a candidate path to be re-computed at a certain point in time.

Security Considerations This document does provide guidance on how to implement a CS-SR Policy leveraging existing mechanisms and protocol extensions. As such, it does not introduce any new security considerations. Security considerations for the SR Policy Architecture defined in do apply to this document. Depending on how a CS-SR Policy is instantiated and reported, the following security considerations do apply

• PCEP:
  • Section 8 of
  • Section 6 of
  • Section 7 of
  • Section 10 of
  • Section 8 of
• BGP:
  • Section 7 of
  • Section 9 of
• Configuration:
  • Section 8 of

Depending on the protocol used for OAM, the following security considerations do apply

• STAMP: Section 15 of and
• BFD: Section 9 of

IANA Considerations This document has no IANA actions.

Acknowledgements The author's want to thank Samuel Sidor, Mike Koldychev, Rakesh Gandhi, Alexander Vainshtein, Tarek Saad, Ketan Talaulikar and Yao Liu for providing their review comments, Yao Liu for her very detailed shepherd review and all contributors for their inputs and support.

References Normative References The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. Although PCEP explicitly makes no assumptions regarding the information available to the PCE, it also makes no provisions for PCE control of timing and sequence of path computations within and across PCEP sessions. This document describes a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. Segment Routing (SR) allows a node to steer a packet flow along any path. Intermediate per-path states are eliminated thanks to source routing. SR Policy is an ordered list of segments (i.e., instructions) that represent a source-routed policy. Packet flows are steered into an SR Policy on a node where it is instantiated called a headend node. The packets steered into an SR Policy carry an ordered list of segments associated with that SR Policy. This document updates RFC 8402 as it details the concepts of SR Policy and steering into an SR Policy. Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Segment Routing (SR) leverages the source-routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. In the MPLS data plane, the SR header is instantiated through a label stack. This document specifies the forwarding behavior to allow instantiating SR over the MPLS data plane (SR-MPLS). Segment Routing (SR) enables any head-end node to select any path without relying on a hop-by-hop signaling technique (e.g., LDP or RSVP-TE). It depends only on "segments" that are advertised by link-state Interior Gateway Protocols (IGPs). An SR path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), an explicit configuration, or a Path Computation Element (PCE). This document specifies extensions to the Path Computation Element Communication Protocol (PCEP) that allow a stateful PCE to compute and initiate Traffic-Engineering (TE) paths, as well as a Path Computation Client (PCC) to request a path subject to certain constraints and optimization criteria in SR networks. This document updates RFC 8408. Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header (SRH). This document describes the SRH and how it is used by nodes that are Segment Routing (SR) capable. Segment Routing (SR) can be used to steer packets through a network using the IPv6 or MPLS data plane, employing the source routing paradigm. An SR Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE). Since SR can be applied to both MPLS and IPv6 data planes, a PCE should be able to compute an SR Path for both MPLS and IPv6 data planes. The Path Computation Element Communication Protocol (PCEP) extension and mechanisms to support SR-MPLS have been defined. This document outlines the necessary extensions to support SR for the IPv6 data plane within PCEP. A Segment Routing (SR) Policy is an ordered list of segments (also referred to as "instructions") that define a source-routed policy. An SR Policy consists of one or more Candidate Paths (CPs), each comprising one or more segment lists. A headend can be provisioned with these CPs using various mechanisms such as Command-Line Interface (CLI), Network Configuration Protocol (NETCONF), Path Computation Element Communication Protocol (PCEP), or BGP. This document specifies how BGP can be used to distribute SR Policy CPs. It introduces a BGP SAFI for advertising a CP of an SR Policy and defines sub-TLVs for the Tunnel Encapsulation Attribute to signal information related to these CPs. Furthermore, this document updates RFC 9012 by extending the Color Extended Community to support additional steering modes over SR Policy. Cisco Systems Cisco Systems Huawei Technologies SoftBank Juniper Networks This document defines a YANG data model for Segment Routing (SR) Policy that can be used for configuring, instantiating, and managing SR policies. The model is generic and applies equally to the MPLS and SRv6 instantiations of SR policies. Individual Cisco Systems Huawei Technologies RtBrick Inc. Nvidia This document describes a mechanism to collect the Segment Routing Policy information that is locally available in a node and advertise it into BGP Link-State (BGP-LS) updates. Such information can be used by external components for path computation, re-optimization, service placement, network visualization, etc. This document specifies the Path Computation Element (PCE) Communication Protocol (PCEP) for communications between a Path Computation Client (PCC) and a PCE, or between two PCEs. Such interactions include path computation requests and path computation replies as well as notifications of specific states related to the use of a PCE in the context of Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering. PCEP is designed to be flexible and extensible so as to easily allow for the addition of further messages and objects, should further requirements be expressed in the future. [STANDARDS-TRACK] This document updates RFC 5440 to clarify usage of the Local Protection Desired bit signaled in the Path Computation Element Communication Protocol (PCEP). This document also introduces a new flag for signaling protection enforcement in PCEP. This document introduces a generic mechanism to create a grouping of Label Switched Paths (LSPs) in the context of a Path Computation Element (PCE). This grouping can then be used to define associations between sets of LSPs or between a set of LSPs and a set of attributes (such as configuration parameters or behaviors), and it is equally applicable to the stateful PCE (active and passive modes) and the stateless PCE. Huawei Technologies Huawei Technologies China Mobile Cisco Systems, Inc. ZTE Corporation The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Clients (PCCs) requests. Segment routing (SR) leverages the source routing and tunneling paradigms. The Stateful PCEP extensions allow stateful control of Segment Routing Traffic Engineering (TE) Paths. Furthermore, PCEP can be used for computing SR TE paths in the network. This document defines PCEP extensions for grouping two unidirectional SR Paths (one in each direction in the network) into a single associated bidirectional SR Path. The mechanisms defined in this document can also be applied using a stateful PCE for both PCE- initiated and PCC-initiated LSPs or when using a stateless PCE. This document defines Path Computation Element Communication Protocol (PCEP) extensions for grouping two unidirectional MPLS-TE Label Switched Paths (LSPs), one in each direction in the network, into an associated bidirectional LSP. These PCEP extensions can be applied either using a stateful PCE for both PCE-initiated and PCC-initiated LSPs or using a stateless PCE. The PCEP procedures defined are applicable to the LSPs using RSVP-TE for signaling. Ciena Corporation Ciena Corporation Cisco Systems, Inc. Juniper Networks, Inc. Huawei Technologies Nokia A Segment Routing (SR) Policy is an ordered list of instructions, called "segments" that represent a source-routed policy. Packet flows are steered into an SR Policy on a node where it is instantiated. An SR Policy is made of one or more candidate paths. This document specifies the Path Computation Element Communication Protocol (PCEP) extension to signal candidate paths of an SR Policy. Additionally, this document updates RFC 8231 to allow delegation and setup of an SR Label Switched Path (LSP), without using the path computation request and reply messages. This document is applicable to both Segment Routing over MPLS (SR-MPLS) and Segment Routing over IPv6 (SRv6). Cisco Systems, Inc. Airtel India Nokia Verizon Huawei Technologies Segment Routing (SR) enables a node to steer packet flows along a specified path without the need for intermediate per-path states, due to the utilization of source routing. An SR Policy comprises a sequence of segments, which are essentially instructions that define a source-routed policy This document proposes a set of extensions to the Path Computation Element Communication Protocol (PCEP) for Segment Routing Policies that are designed to satisfy requirements for connection-oriented transport services (Circuit-Style SR policies). They include the ability to control path recomputation and the option to request path with strict hops only and are also applicable for generic SR policy use cases where controlling path recomputation or distinct hop requirements are applicable. Ciena Corporation Ciena Corporation Cisco Systems Juniper Networks, Inc. Nokia Ciena Huawei Technologies Verizon Inc. Cisco Systems. Certain traffic engineering path computation problems require solutions that consist of multiple traffic paths that together form a solution. Returning a single traffic path does not provide a valid solution. This document defines mechanisms to encode multiple paths for a single set of objectives and constraints. This allows encoding of multiple Segment Lists per Candidate Path within a Segment Routing Policy. The new Path Computation Element Communication Protocol (PCEP) mechanisms are designed to be generic, where possible, to allow for future re-use outside of SR Policy. The new PCEP mechanisms are applicable to both stateless and stateful PCEP. Additionally, this document updates RFC 8231 to allow encoding of multiple Segment Lists in PCEP. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. The extensions for stateful PCE provide active control of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs) via PCEP, for a model where the PCC delegates control over one or more locally configured LSPs to the PCE. This document describes the creation and deletion of PCE-initiated LSPs under the stateful PCE model. This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. [STANDARDS-TRACK] This document introduces a simple mechanism to associate a group of Label Switched Paths (LSPs) via an extension to the Path Computation Element Communication Protocol (PCEP) with the purpose of computing diverse (disjointed) paths for those LSPs. The proposed extension allows a Path Computation Client (PCC) to advertise to a Path Computation Element (PCE) that a particular LSP belongs to a particular Disjoint Association Group; thus, the PCE knows that the LSPs in the same group need to be disjoint from each other. Cisco Systems, Inc. Cisco Systems, Inc. Colt Huawei Nokia Segment Routing (SR) leverages the source routing paradigm. SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) data planes. This document describes the procedures for Performance Measurement in SR-MPLS networks using the Simple Two-Way Active Measurement Protocol (STAMP), as defined in RFC 8762, along with its optional extensions defined in RFC 8972 and further augmented in RFC 9503. The described procedure is used for SR-MPLS paths (including SR-MPLS Policies, SR-MPLS IGP best paths, and SR- MPLS IGP Flexible Algorithm paths), as well as Layer-3 and Layer-2 services over the SR-MPLS paths. Cisco Systems, Inc. Cisco Systems, Inc. Colt Huawei Nokia Segment Routing (SR) leverages the source routing paradigm. SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) data planes. This document describes the procedures for Performance Measurement for SRv6 using the Simple Two-Way Active Measurement Protocol (STAMP), as defined in RFC 8762, along with its optional extensions defined in RFC 8972 and further augmented in RFC 9503. The described procedure is used for links and SRv6 paths (including SRv6 Policies, SRv6 IGP best paths, and SRv6 IGP Flexible Algorithm paths), as well as Layer-3 and Layer-2 services over the SRv6 paths. Huawei Technologies Huawei Technologies China Telecom China Mobile Cisco Systems A Segment Routing(SR) policy identifies a set of candidate SR paths Each SR path is passed in BGP as the SR Policy SAFI NLRI accompanied with the Tunnel Encapsulation attribute (Tunnel-encaps). Each SR Path (tunnel) uses a set of TLVs in the Tunnel-encaps attribute to describe the characteristics of the SR Policy tunnel. One of the TLVs that describes the tunnel is the Segment list TLV which provides a list of segments contained in the tunnel. This document specifies a new Path Segment Sub-TLV to associate a Path Segment ID to the SR Segment List. The Path Segment ID can be used for performance measurement, path correlation, and end-2-end path protection. This Path Segment identifier can be also be used to correlate two unidirectional SR paths into a bidirectional SR path. Bidirection SR path may be required in some scenarios such as mobile backhaul transport network. Informative References This document defines a common terminology for Generalized Multi-Protocol Label Switching (GMPLS)-based recovery mechanisms (i.e., protection and restoration). The terminology is independent of the underlying transport technologies covered by GMPLS. This memo provides information for the Internet community. IEEE An Ethernet pseudowire (PW) is used to carry Ethernet/802.3 Protocol Data Units (PDUs) over an MPLS network. This enables service providers to offer "emulated" Ethernet services over existing MPLS networks. This document specifies the encapsulation of Ethernet/802.3 PDUs within a pseudowire. It also specifies the procedures for using a PW to provide a "point-to-point Ethernet" service. [STANDARDS-TRACK] This document describes a pseudowire encapsulation for Time Division Multiplexing (TDM) bit-streams (T1, E1, T3, E3) that disregards any structure that may be imposed on these streams, in particular the structure imposed by the standard TDM framing. [STANDARDS-TRACK] This document expands the applicability of Virtual Private Wire Service (VPWS) bit-stream payloads beyond Time Division Multiplexing (TDM) signals and provides pseudowire transport with complete signal transparency over Packet Switched Networks (PSNs). This document describes a method for encapsulating structured (NxDS0) Time Division Multiplexed (TDM) signals as pseudowires over packet-switching networks (PSNs). In this regard, it complements similar work for structure-agnostic emulation of TDM bit-streams (see RFC 4553). This memo provides information for the Internet community. This document provides encapsulation formats and semantics for emulating Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) circuits and services over MPLS. [STANDARDS-TRACK] The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header. Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet. This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization. Individual Cisco Systems Cisco Systems INSA Lyon Orange Bell Canada This document presents Topology Independent Loop-free Alternate Fast Reroute (TI-LFA), aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This Fast Reroute (FRR) behavior builds on proven IP Fast Reroute concepts being LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding (DLFA). It extends these concepts to provide guaranteed coverage in any two-connected networks using a link-state IGP. An important aspect of TI-LFA is the FRR path selection approach establishing protection over the expected post-convergence paths from the point of local repair, reducing the operational need to control the tie-breaks among various FRR options. This document defines a PHB (per-hop behavior) called Expedited Forwarding (EF). The PHB is a basic building block in the Differentiated Services architecture. EF is intended to provide a building block for low delay, low jitter and low loss services by ensuring that the EF aggregate is served at a certain configured rate. This document obsoletes RFC 2598. [STANDARDS-TRACK] This document defines a general use Differentiated Services (DS) Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF). [STANDARDS-TRACK] There are deployments where the Layer 3 interface on which IS-IS operates is a Layer 2 interface bundle. Existing IS-IS advertisements only support advertising link attributes of the Layer 3 interface. If entities external to IS-IS wish to control traffic flows on the individual physical links that comprise the Layer 2 interface bundle, link attribute information about the bundle members is required. This document introduces the ability for IS-IS to advertise the link attributes of Layer 2 (L2) Bundle Members. There are deployments where the Layer 3 (L3) interface on which OSPF operates is a Layer 2 (L2) interface bundle. Existing OSPF advertisements only support advertising link attributes of the L3 interface. If entities external to OSPF wish to control traffic flows on the individual physical links that comprise the L2 interface bundle, link attribute information for the bundle members is required. This document defines the protocol extensions for OSPF to advertise the link attributes of L2 bundle members. The document also specifies the advertisement of these OSPF extensions via the Border Gateway Protocol - Link State (BGP-LS) and thereby updates RFC 9085. Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). This document describes the IS-IS extensions that need to be introduced for Segment Routing operating on an MPLS data plane. Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). This document describes the OSPFv2 extensions required for Segment Routing. Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological subpaths, called "segments". These segments are advertised by routing protocols, e.g., by the link-state routing protocols (IS-IS, OSPFv2, and OSPFv3) within IGP topologies. This document defines extensions to the Border Gateway Protocol - Link State (BGP-LS) address family in order to carry SR information via BGP. The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called "segments". It can be implemented over the MPLS or the IPv6 data plane. This document describes the IS-IS extensions required to support SR over the IPv6 data plane. This document updates RFC 7370 by modifying an existing registry. The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called segments. It can be implemented over an MPLS or IPv6 data plane. This document describes the OSPFv3 extensions required to support SR over the IPv6 data plane. Segment Routing over IPv6 (SRv6) allows for a flexible definition of end-to-end paths within various topologies by encoding paths as sequences of topological or functional sub-paths called "segments". These segments are advertised by various protocols such as BGP, IS-IS, and OSPFv3. This document defines extensions to BGP - Link State (BGP-LS) to advertise SRv6 segments along with their behaviors and other attributes via BGP. The BGP-LS address-family solution for SRv6 described in this document is similar to BGP-LS for SR for the MPLS data plane, which is defined in RFC 9085. Cisco Systems Cisco Systems Cisco Systems Orange INSA Lyon Cisco Systems This document presents a mechanism aimed at providing loop avoidance in the case of IGP network convergence event. The solution relies on the temporary use of SR policies ensuring loop-freeness over the post-convergence paths from the converging node to the destination. This document defines a way for an Intermediate System to Intermediate System (IS-IS) router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment ID (SID) stack can be supported in a given network. This document only defines one type of MSD: Base MPLS Imposition. However, it defines an encoding that can support other MSD types. This document focuses on MSD use in a network that is Segment Routing (SR) enabled, but MSD may also be useful when SR is not enabled. This document defines a way for an Open Shortest Path First (OSPF) router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network. This document only refers to the Signaling MSD as defined in RFC 8491, but it defines an encoding that can support other MSD types. Here, the term "OSPF" means both OSPFv2 and OSPFv3. This document defines a way for a Border Gateway Protocol - Link State (BGP-LS) speaker to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network. This document describes a protocol intended to detect faults in the bidirectional path between two forwarding engines, including interfaces, data link(s), and to the extent possible the forwarding engines themselves, with potentially very low latency. It operates independently of media, data protocols, and routing protocols. [STANDARDS-TRACK] This document defines Seamless Bidirectional Forwarding Detection (S-BFD), a simplified mechanism for using BFD with a large proportion of negotiation aspects eliminated, thus providing benefits such as quick provisioning, as well as improved control and flexibility for network nodes initiating path monitoring. This document updates RFC 5880.

Contributors Bell Canada

daniel.voyer@bell.ca

Verizon

luay.jalil@verizon.com

Huawei Technologies

pengshuping@huawei.com

Cisco Systems, Inc.

cfilsfil@cisco.com

Cisco Systems, Inc.

fclad@cisco.com

Cisco Systems, Inc.

tsaad.net@gmail.com

Cisco Systems, Inc.

brfoster@cisco.com

Cisco Systems, Inc.

bduvivie@cisco.com

Cisco Systems, Inc.

slitkows@cisco.com

Huawei Technologies

jie.dong@huawei.com